In conventional methods for running structural casing in offshore drilling operations, a temporary guide base is lowered on guide lines from the surface of the water and placed on the ocean floor. An opening through the center of the guide base is positioned over the site of the well to be drilled and serves as a re-entry means to the well site. A string of drill pipe having a drill bit on the lower end thereof is then lowered through the opening in the guide base and a hole is drilled into the ocean floor to the setting depth of the structural pipe string. The guide lines are used to guide the drill string to the opening in the guide base. The drill pipe is removed from the drilled hole to the water's surface. Structural casing is then lowered through the opening in the temporary guide base and into the drilled hole, guided by the guide lines, on drill pipe by means of a suitable running tool connected to the lower end of the drill pipe and releasably connected to the structural casing. A permanent guide base is attached to the upper end of the casing pipe. Once the structural casing has been set it is cemented in place. The running rool is released from the structural casing string and removed along with the drill string to the water's surface.
In a location that has a soft unstable, unconsolidated ocean floor the temporary guide base may settle below the ocean floor rendering it useless as a reentry means. For this reason and also, because of other problems such as, severe hole instability and loss of drilling fluid (used to control formation pressure and to clean and stabilize the well bore) into the formation, this method for running and cementing structural casing strings is not satisfactory in such locations.
The present invention overcomes these problems by allowing the structural casing string to be cemented in place prior to releasing it from the drill pipe running string thereby eliminating the need for drilling a hole prior to running the structural casing string. Further, the invention eliminates the need for running a temporary guide base. This feature makes this invention also advantageous for use in normal, firm bottom water locations. Considerable rig time is saved by eliminating the running of a temporary guide base when running pipe in either type location.